939 research outputs found
Supersoft X-ray sources in M31: II. ROSAT-detected supersoft sources in the ROSAT, Chandra and XMM eras
We have performed Chandra observations during the past 3 years of 5 of the
M31 supersoft X-ray sources (SSS) discovered with ROSAT. Surprisingly, only one
of these sources has been detected, despite a predicted detection of about
20-80 counts. This has motivated a thorough check of the ROSAT M31 survey I
data, including a relaxation of the hardness ratio requirement used to select
SSS. This increases the number of SSS identified in survey I by 7. We then
carried out a comparison with the ROSAT M31 survey II dataset which had
hitherto not been explicitly investigated for SSS. We find that most of the
ROSAT survey I sources are not detected, and only two new SSS are identified.
The low detection rate in the ROSAT survey II and our Chandra observations
implies that the variability time scale of SSS is a few months. If the majority
of these sources are close-binary SSS with shell hydrogen burning, it further
implies that half of these sources predominantly experience large mass transfer
rates.Comment: accepted for publ. in ApJ; 2 ps-figures; high-quality figures
available at http://www.mpe.mpg.de/~jcg/publis.htm
A new clue to the transition mechanism between optical high and low states of the supersoft X-ray source RX J0513.9-6951, implied from the recurrent nova CI Aquilae 2000 outburst model
We have found a new clue to the transition mechanism between optical
high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray
source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000
outburst. These drops are naturally attributed to cessation of optically thick
winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that
the size of a disk drastically shrinks when the wind stops. This causes ~1-2
mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism
reproduces sharp ~1 mag drop from optical high to low states. We predict this
mechanism also works on the transition from low to high states. Interaction
between the wind and the companion star attenuates the mass transfer and drives
full cycles of low and high states.Comment: 9 pages including 5 figures, to appear in the Astrophysical Journa
The ROSAT Galactic Plane Survey: analysis of a low latitude sample area in Cygnus
We present the analysis of the point source content of a low galactic
latitude region selected from the ROSAT all-sky survey. The test field is
centered at l = 90deg, b = 0deg and has an area of 64.5 deg2. A total of 128
soft X-ray sources are detected above a maximum likelihood of 8. Catalogue
searches and optical follow-up observations show that in this direction of the
galactic plane, 85% of the sources brighter than 0.03 PSPC cts/s are identified
with active coronae. F-K type stars represent 67% (+/-13%) of the stellar
identifications and M type stars account for 19% (+/- 6%). These results
together with those of similar optical campaigns demonstrate that the soft
X-ray population of the Milky Way is largely dominated by active stars. The
modelling of this population suggests that most of the stars detected by ROSAT
in this direction are younger than 1 Gyr. The small number of unidentified
sources at low X-ray flux put rather strong constraints on the hypothetical
X-ray emission from old neutron stars accreting from the interstellar medium.
Our observations clearly rule out models which assume no dynamical heating for
this population and a total number of Nns = 10^9 neutron stars in the Galaxy.
If accretion on polar caps is the dominant mode then our upper limit may imply
Nns ~ 10^8.Comment: 27 pages, 22 figures, Latex. Full resolution PostScript available at
ftp://cdsarc.u-strasbg.fr/pub/xray/rgps_cygnus.ps.gz To be published in
Astronomy & Astrophysics, Main Journa
VLT observations of GRB 990510 and its environment
We present BVRI photometry and spectrophotometry of GRB990510 obtained with
the ESO VLT/Antu telescope during the late decline phase. Between days 8 and 29
after the burst, the afterglow faded from R=24.2 to ~26.4. The spectral flux
distribution and the light curve support the interpretation of the afterglow as
synchrotron emission from a jet. The light curve is consistent with the optical
transient alone but an underlying SN with maximum brightness R>27.4 or a galaxy
with R>27.6 (3-sigma upper limits) cannot be ruled out. To a 5-sigma detection
threshold of R=26.1, no galaxy is found within 6'' of the transient. A very
blue V~24.5 extended object which may qualify as a starburst galaxy is located
12'' SE, but at unknown redshift.Comment: 5 pages A&A Latex, accepted for publication in A&A Letter
Identification of selected sources from the ROSAT Galactic Plane Survey - I
We report on optical searches in the error circles of 93 ROSAT survey sources
located at low galactic latitudes (|b| < 20 deg). These sources were extracted
from the ROSAT Galactic Plane Survey using various selection criteria on
hardness ratio, X-ray and optical brightness and integrated galactic absorption
in the direction of the source. We find optical identifications in 76 cases,
among which are 25 new AGN, 6 new CVs and a new Be/X-ray binary. In order to
illustrate the relevance of the source selections applied here, we
cross-correlated the ROSAT all-sky survey bright source list with SIMBAD.
Different classes of X-ray emitters populate distinct regions of a multi
dimensional parameter space involving flux ratios, galactic latitude and Nh.
This relatively good segregation offers the possibility to build source samples
with enhanced probability of identification with a given class. Complete
optical identification of such subsamples could eventually be used to compute
meaningful probabilities of identification for all sources using as basis a
restricted set of multi-wavelength information.Comment: 18 pages, 7 figures, Postscript. Optical spectra and finding charts
available at ftp://cdsarc.u-strasbg.fr/pub/xray/rgps_idI.ps.gz Submitted to
Astronomy & Astrophysics, Supplement Serie
Testing Mode-Coupling Theory for a Supercooled Binary Lennard-Jones Mixture II: Intermediate Scattering Function and Dynamic Susceptibility
We have performed a molecular dynamics computer simulation of a supercooled
binary Lennard-Jones system in order to compare the dynamical behavior of this
system with the predictions of the idealized version of mode-coupling theory
(MCT). By scaling the time by the temperature dependent -relaxation
time , we find that in the -relaxation regime and
, the coherent and incoherent intermediate scattering functions, for
different temperatures each follows a -dependent master curve as a function
of scaled time. We show that during the early part of the -relaxation,
which is equivalent to the late part of the -relaxation, these master
curves are well approximated by the master curve predicted by MCT for the
-relaxation. This part is also fitted well by a power-law, the so-called
von Schweidler law. We show that the effective exponent of this power-law
depends on the wave vector if is varied over a large range. The early
part of the -relaxation regime does not show the critical decay
predicted by MCT. The -dependence of the nonergodicity parameter for
and are in qualitative agreement with MCT. On the time
scale of the late -relaxation the correlation functions show a
Kohlrausch-Williams-Watt behavior (KWW). The KWW exponent is
significantly different from the effective von Schweidler exponent . At low
temperatures the -relaxation time shows a power-law behavior
with a critical temperature that is the same as the one found previously for
the diffusion constant [Phys. Rev. Lett. {\bf 73}, 1376 (1994)]. The critical
exponent of this power-law and the von Schweidler exponent fulfill the
connection proposed by MCT between these two quantities. We also show that theComment: 28 Pages of REVTEX, Figures available from W. Ko
RX J0513.9-6951: The first example of accretion wind evolution, a key evolutionary process to Type Ia supernovae
A new self-sustained model for long-term light curve variations of RX
J0513.9-6951 is proposed based on an optically thick wind model of
mass-accreting white dwarfs (WDs). When the mass accretion rate to a WD exceeds
the critical rate of \sim 1 x 10^{-6} M_\sun yr^{-1}, optically thick strong
winds begin to blow from the WD so that a formation of common envelope is
avoided. The WD can accrete and burn hydrogen-rich matter atop the WD at the
critical rate. The excess matter transferred to the WD above the critical rate
is expelled by winds. This is called the accretion wind evolution. This
ejection process, however, occurs intermittently because the mass transfer is
attenuated by strong winds: the strong winds collide with the secondary surface
and strip off the very surface layer of the secondary. The matter stripped-off
is lost from the binary system. Properly formulating this mass stripping effect
and the ensuing decay of mass transfer rate, we are able to reproduce, in a
self-sustained manner, the transition between the optical high/X-ray off and
optical low/X-ray on states of RX J0513.9-6951. Thus RX J0513.9-6951 is the
first example of the accretion wind evolution, which is a key evolutionary
process in a recently developed evolutionary path to Type Ia supernovae.Comment: 22 pages including 13 figures, to appear in the Astrophysical Journa
Programmable active memories in real-time tasks: implementing data-driven triggers for LHC experiments
The future Large Hadron Collider (LHC), to be built at CERN, presents among other technological challenges a formidable problem of real-time data analysis. At a primary event rate of 40 MHz, a multi-stage trigger system has to analyze data to decide which is the fraction of events that should be preserved on permanent storage for further analysis. We report on implementations of local algorithms for feature extraction as part of triggering, using the detectors of the proposed ATLAS experiment as a model. The algorithms were implemented for a decision frequency of 100 kHz, on different data-driven programmable devices based on structures of field- programmable gate arrays and memories. The implementations were demonstrated at full speed with emulated input, and were also integrated into a prototype detector running in a test beam at CERN, in June 1994
VLT spectroscopy of GRB 990510 and GRB 990712; probing the faint and bright end of the GRB host galaxy population
We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first epoch GRB 990510 spectrum, we determine the redshift for this burst at z >= 1.619. No clear emission lines are detected. The strength of the MgI feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint (V >~ 28), the GRB afterglow allows us to probe its interstellar medium and - in principle - to measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts), shows clear features both in emission and absorption, at a redshift of z = 0.4331 +/- 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an HII galaxy. We derive a unreddened [OII] star formation rate of 2.7 +/- 0.8 Msun/yr. Correcting for the measured extinction intrinsic to the host galaxy (A(V) = \gpm{3.4}{2.4}{1.7}), this value increases to \gpm{35}{178}{25} Msun/yr. The [OII] equivalent width, compared to that of field galaxies at z <= 1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and Hbeta emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log(O/H) = -3.7 +- 0.4, which is slightly below the lowest metallicity one finds in nearby spiral galaxies. For both GRBs we study the time evolution of the absorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We find no evidence for a significant change in the MgII width
Low Concentrations of Silver Nanoparticles in Biosolids Cause Adverse Ecosystem Responses under Realistic Field Scenario
A large fraction of engineered nanomaterials in consumer and commercial products will reach natural ecosystems. To date, research on the biological impacts of environmental nanomaterial exposures has largely focused on high-concentration exposures in mechanistic lab studies with single strains of model organisms. These results are difficult to extrapolate to ecosystems, where exposures will likely be at low-concentrations and which are inhabited by a diversity of organisms. Here we show adverse responses of plants and microorganisms in a replicated long-term terrestrial mesocosm field experiment following a single low dose of silver nanoparticles (0.14 mg Ag kgâ1 soil) applied via a likely route of exposure, sewage biosolid application. While total aboveground plant biomass did not differ between treatments receiving biosolids, one plant species, Microstegium vimeneum, had 32 % less biomass in the Slurry+AgNP treatment relative to the Slurry only treatment. Microorganisms were also affected by AgNP treatment, which gave a significantly different community composition of bacteria in the Slurry+AgNPs as opposed to the Slurry treatment one day after addition as analyzed by T-RFLP analysis of 16S-rRNA genes. After eight days, N2O flux was 4.5 fold higher in the Slurry+AgNPs treatment than the Slurry treatment. After fifty days, community composition and N2O flux of the Slurry+AgNPs treatment converged with the Slurry. However, the soil microbial extracellular enzymes leucine amino peptidase and phosphatase had 52 and 27% lower activities, respectively, while microbial biomass was 35% lower than the Slurry. We also show that the magnitude of these responses was in all cases as large as or larger than the positive control, AgNO3, added at 4-fold the Ag concentration of the silver nanoparticles
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